POWDER METALLURGY-- Compacting-1.2 Cold Isostatic Pressing

1.2 Cold Isostatic Pressing. ln isostatic pressing, the powder is filled into filexible molds made of rubber or elastomers and subjected-to hydrostatic pressure. The pressure is commonly in the range of 200 t0 400 MPa. As a result of the uniform pressure, a much higher uniformity in density is achieved. Isostatic pressing has gained much importance during the last 30 years, because it offers several further important advantages compared to rigid die pressing:

*Lower pressure required for a certain green density.

*Higher strength of the compacts.

*More free choice in dimension (ratio of diameter to lengths).

*Parts with undercuts and reentrant angles can be pressed.

*Thin-walled tubes can be produced.

*Very Iarge parts can be compacted.

Less precise dimensional control and a much lower rate of production are the two main disadvantages.

Isostatic pressing is carried out by two different techniques: wetbag pressing or drybag pressing.  In wetbag pressing, the powder is filled into flexible molds which are sealed outside the pressure vessel. Several molds (either of the same shape or of different shape) are then immersed in a fluid, most commonly water, and pressure is applied isostatically. Wetbag pressing is the most common technique for producing forging or rolling performs, where an even and high green density is more important than dimensional control. Nevertheless, wetbag pressing is also used for more complex geometries and even near-net shape parts. The size of the part is limited by the size of the pressure vessel. Tungsten ingots of up to 1000kg are produced via wetbag pressing.

In drybag pressing, the elastomeric mold is fixed into the pressure vessel. The mold is filed with powder and sealed with a cover plate. Then pressure is applied between the mold and the vessel wall. After pressure release, the cover is removed and the part removed. Then the procedure starts again. Pressure Vessels with both top and bottom plate are in use, and allow more rapid removal of compact.

Drybag pressing is used for sample shapes, such as plugs, and high production rates (mass production). However, only one compact (with one specific shape) can be pressed at a time.